I wanted to add an outlet on the wall over my workbench. I tapped into a 20amp line at the outlet. Only problem is that both the upper and lower screws were filled with the black/white and ground wires. I piggy backed on one anyway. Eveything seems to be OK. Am I doing anything wrong??

It should be OK, but proper way to do it (providing box is deep enough to accommodate everything,may need box extention) sounds like you have a line in and line out, so take the blacks and add 2 black pigtails and wire nut them all together. Do the same with whites. Grounds can be twisted together far enough up the wires and pinched together with a small slip on ground clamp (lowes or h.d. have them)and you should have 2ground tails left to tie into each plug. Then wire 1 black and 1 white pigtail to each plug.You could run just 1 pigtail out of wire nuts and just put small jumper from 1 plug to the other. But I like to keep them separate in case I ever have to work on one. Sounds more involved than it is. Hope this helps. D.